This invention relates to data compression and expansion and in particular, to a method and device for real time speech compression and expansion. While the device is described in conjunction with an announcement system whose function is compatible with telephone lines, those skilled in the art will recognize the wider applicability of the inventive principles disclosed hereinafter.
A number of digital compressors and expanders have been developed in the prior art. In general, these devices operate to reduce the number of bits requiring transmission in a data stream from the processor to an associated expanding device, with or without intermediate storage. Examples of this form of processor are shown and described in the United States Patents to Duttweiler et al, U.S. Pat. No. 3,945,002, dated Mar. 16, 1976, and references cited therein. Other system forms are designed to determine majority binary value in particular groups and that majority value is further utilized in the system. An example of this later design concept is disclosed in the United States Patent to Armstrong, U.S. Pat. No. 3,696,402, dated Oct. 3, 1972, and references cited therein.
While the Duttweiler and Armstrong patents in particular, and the prior art in general, work well for their intended purposes, they do not lend themselves to use in the particular system utilization with which the invention disclosed hereinafter is directed. The system with which my invention initially found utility is such that real time speech compression and expansion is mandatory. In addition, the resulting decompression should have a quality nearly equal to a recording. The technique developed in view of this need enables speech data, sampled at an eight K hertz rate, consisting of eight bits per sample, to be compressed in real time by a factor of approximately 2:1, and upon decompression, be essentially indistinguishable from the original digitized speech data. Decompression is accomplished simply and with minimum hardware.
In general, the technique used in compressing the speech data involves digitizing the data, determining the maximum amplitude of the digitized data over a given short period, compressing the data by dividing the largest amplitude constant over this short period into the digitized data of the corresponding short period, and storing the results on some suitable storage medium. To decompress the data, the data is accessed from the storage medium and the constant inserted. Thereafter, the data may be converted from digital to analog form for furtheruse. The particular system with which the compression and decompression scheme finds application also employs a novel memory means for sample data storage.
One of the objects of this invention is to provide an improved technique for compressing audio data.
Another object of this invention is to provide a compression scheme for audio data which reduces the storage requirement of the data.
Another object of this invention is to provide a low cost storage device for audio data.
Other objects will be apparent to those skilled in the art in light of the following description and accompanying drawings.